Heat transfer material for an improved die edge contacting socket

ABSTRACT

An improved die edge contacting socket incorporates particles of a thermally conducting material into an elastomeric compression pad disposed in the sealing cap of the socket. The elastomeric compression pad is preferably composed of an electrically insulating material, such as a silicone-based gel. The thermally conducting material is preferably either diamond, beryllium oxide, silicon nitride, or a like material.

This application is a divisional of application Ser. No. 08/716,084,filed Sep. 19, 1996 now U.S. Pat. No. 6,252,302.

FIELD OF THE INVENTION

The present invention relates to packaging of semiconductor integratedcircuits and, more particularly, to a multi-die encapsulation device.

BACKGROUND OF THE INVENTION

Modem semiconductor computing systems have often utilized sets ofindividually packaged semiconductor dice mounted and interconnected on acircuit board. More recent designs have eliminated the numerousindividual die packages in favor of a single package cable of housingseveral bare dice. The advantages of such systems are greater computingpower per unit area of circuit board, and lower packaging cost. FIG. 1shows a portion of an example of such a conventional multi-dice package,illustrated in a cross-sectional view. The dice are intially formed inlarge groups on wafers. By cutting the wafer, the dice are individuallysegregated. The encapsulation device is typically molded plastic and hasa chamber portion comprised of a plurality of die chambers 5. Eachchamber has at least one beveled edge 10. A bare die is inserted by handinto a chamber 5 with the circuit side touching the beveled edge 10. Thebeveled edge 10 thus serves as a guide for the insertion of the baredie. However, since the circuit slides across the bevel, the circuitrymay be damaged during insertion. The bare die is retained by aspring-retaining and contact assembly 15 located at the bottom of thechamber 5. The retaining and contact assembly 15 holds the bare die inposition in the encapsulation device, with a spring portion 20electrically contacting the bare die.

A rigid foot portion 25 is provided for contacting a circuit board ontowhich the encapsulation device is mounted. Due to the rigidity of thefoot portion 25 and inherent bowing of many circuit boards, the failurerate of electrical contact between the bare dice and the board istypically high. At times the failure rate runs as high as 80%.

Heat dissipation is a persistent problem in the packaging systems ofvirtually all semiconductor devices, including the encapsulation deviceshown in FIG. 1. Long-term exposure to excessive temperatures may impedethe operation of a die or lead to an electrical failure. There arevarious approaches available to lessen the problem of die heating thatinvolve either redesigning the die circuitry or modifying theencapsulation device. For instance, designing circuits to operate atlower voltage levels may provide a partial solution. However, loweroperating voltages may not be possible for a given die.

Alternatively, certain features may be incorporated into theencapsulation device itself to improve heat transfer. The encapsulationdevice shown in FIG. 1 provides few pathways for the transfer of heatfrom the dice. This is due to the relatively small amount of physicalcontact between the encapsulation device and the dice and to the lessthan optimal thermoconductivity for molded plastic. Although there willbe some minimal amount of natural convective heat transfer between thedice and the ambient, the amount is of little consequence. Further,radiative heat transfer does not ordinarily play a significant rolebecause the temperatures required for significant radiative heattransfer from the dice are normally higher than the maximum permissibleoperating temperature of the dice.

To improve heat transfer from the dice, a cap, of the type to bedescribed below, may be placed on the encapsulation device shown in FIG.1, and may be modified in several ways, depending on the heat output ofthe dice. For a relatively low heat output combination of dice, the capmay be made of metal to improve conduction from the die chambers 5. Ifadditional heat transfer capacity is necessary, the cap may be fittedwith slots, and possibly a forced air supply, to improve convection. Thecap may be also be provided with fins to improve both the conduction andconvection. The addition of fins, slots, and fans increases thecomplexity of the device, and space limitations may rule out their use.Furthermore, even with the aforementioned modifications, the predominantheat transfer mechanism will continue to be convection, which is lessefficient than conduction.

In addition to these problems, solid caps secured over the chamberportion of the encapsulation device may not retain the dice in thecorrect position and often are a cause of dice damage subsequent toencapsulation of the dice.

Thus, a need exists for an encapsulation device for bare dice whichprovides reliable electrical contact between the dice and a mountingboard and a need exists for a method for safely inserting the bare diceinto the encapsulation device. In addition, there is a need to provideposition retainment of the bare dice within the encapsulation devicewithout fear of dice damage following encapsulation, and a need toincorporate conductive heat dissipation features into the encapsulationdevice without the need for space limiting heat sinks or circuit designchanges to accommodate lower voltages.

SUMMARY OF THE INVENTION

In one aspect of the present invention, an encapsulation device isprovided. The encapsulation device has a chamber portion that includes adie chamber that is operable to accept one bare die. There is also a padof elastomeric material containing particles of a thermally conductingmaterial for biasing the bare dice in the die chamber.

In another aspect of the present invention, a cap for mating with andsealing an encapsulation device that is operable to house bare die isprovided. The cap includes an elastomeric compression pad for biasingthe bare die in a correct orientation when the bare die is inserted intoa chamber in the encapsulation device. The elastomeric material containsparticles of a thermally conducting material.

In still another aspect of the present invention, an encapsulationdevice is provided. The encapsulation device includes a chamber portioncomprising a plurality of parallel die chambers. Each of the pluralityof die chambers is operable to accept one bare die of a plurality ofbare die. A cap portion for mating with and sealing a top portion of thechamber portion is provided. The cap portion comprises an elastomericcompression pad for biasing the bare die in a correct orientation whenthe bare die is inserted into a chamber. The elastomeric compression padcontains particles of a thermally conducting material.

In a further aspect of the present invention, a method of forming anencapsulation device is provided. In the method, a chamber portion isprovided that includes a plurality of die chambers. Each of theplurality of die chambers is operable to accept one bare die. A capportion is provided for mating with and sealing a top portion of thechamber portion. The cap portion is provided with a thermally conductingelastomeric compression pad for biasing the bare die in a givenorientation when the bare die is inserted into a chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of nonlimitative embodiments, with reference tothe attached drawings, wherein below:

FIG. 1 is a simplified encapsulation device of the related art.

FIG. 2 is a three-dimensional view of the encapsulation device of theinvention prior to insertion of bare die and prior to final assembly.

FIG. 3 is a simplified schematic of a bare die and the die pads thereon.

FIG. 4 is a cross-sectional view of one chamber of the encapsulationdevice of the invention with a bare die inserted and seated therein.

FIG. 5 is a top view of one chamber of the encapsulation device of theinvention with a bare die inserted therein.

FIG. 6 is a three-dimensional view of a bare die insertion tool of theinvention.

FIG. 7 is a side view of the three-dimensional view of the bare dieinsertion tool of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a three-dimensional view of the encapsulation device 45 of theinvention prior to insertion of bare die into the encapsulation device.A chamber portion 50 comprises a plurality of chambers 55 for acceptingand retaining the bare dice. The exact number of chambers 55 may varydepending on design considerations. Each chamber 55 comprises retainingedges 60 which define a chamber void 65. The chamber void 65 accepts abare die insertion tool during the process of inserting the bare dieinto each chamber 55.

When the bare dice have been inserted into the chambers 55, a cap 70 issealed to the chamber portion. The cap 70 rests on lip 75 of the chamberportion 50. The final locking seal between the cap 70 and the lip 75 istypically an epoxy which when heated allows the cap 70 to be removed andresealed, thereby allowing for the removal and the replacement of faultydice. However, other locking seals, such as a snap seal, a grooved seal,or a pressure seal, may work equally well. The chamber portion 50 andthe cap 70 are typically a molded plastic, such as, for example, glassfilled polysulfone, polyethersulfone (sometimes referred toalternatively as “PES”), polyamidimide (sometime referred toalternatively as “PAI” or Porlon®), polyphenyline-sulfide, hightemperature PCT polyester, or a like plastic.

FIG. 3 is representative of a single bare die 76 designed to fit intothe encapsulation device 45. In this case the single die 76 has eightdie pads 77, although it is possible for the bare dice to have anynumber of die pads.

FIG. 4 is a cross-sectional view of a single die chamber 55 havingparallel walls 78. A bare die 76 inserted into the die chamber 55 isretained against the retaining edges 60 by retaining contact 85. Theretaining contact 85 electrically contacts one of the die pad 77 of baredie 76 (see FIG. 3) at a spring portion 86 and electrically contacts acircuit board 90 with a compliant foot portion 95 when the bare die 76is seated in the die chamber 55. Thus, the retaining contact 85protrudes through a bottom portion 96 of the chamber portion 50. Thecompliant contact foot 95 provides reliable contact to the circuit board90 even though the circuit board 90 may be bowed. It can be seen fromFIG. 4 that the compliant foot 95 has a vertical displacement d.Therefore, as long as the distance between the reference foot 100 andthe circuit board 90 is less than d, and the distance accommodates awidth of the compliant foot 95, the compliant foot 95 makes contact withthe circuit board 90. The number of retaining contacts 85 is equal tothe number of die pads 77 on the bare die 76. Therefore, for the baredie of FIG. 2, there would be eight retaining contacts 85 in a diechamber 55.

FIG. 5 is a top view of a single die chamber 55. In addition to elementspreviously named, a pair of opposed parallel walls 101 is shown in FIG.5. Preferably, the chamber portion 50 shown in FIG. 1 is oriented duringbare die insertion, such that the chambers 55 are stacked. Thisorientation allows for a simplified insertion process which can best beunderstood with reference to FIGS. 3, 4, 5, 6, and 7.

A bare die 76 is placed on an insertion tool 105, shown threedimensionally in FIG. 6 and cross-sectionally in FIG. 7, with thecircuit side of the bare die 76 up, thereby preventing damage to thecircuitry of the bare die 76. The insertion tool 105 is then insertedinto the chamber void 65 between the two retaining edges 60. The guideedge 110 of the insertion tool 105 is used to mechanically force thebare die 76 into the retaining contact 85 (see FIG. 4). The bare die 76is held in a correct orientation on the insertion tool 105 with a vacuumprovided by a vacuum source (not shown). The vacuum source is connectedto the insertion tool 105 at vacuum connection 115 through a means suchas a plastic hose (not shown). Internal passages 120 connected to thevacuum connection 115 directs the vacuum to suction depressions 125.Thus the die insertion tool 105 allows for insertion of the bare die 76into chamber 55 without damage to die circuitry.

Not only does the process and encapsulation device of the inventionprevent damage to die circuitry, but there is also a space savings overthe beveled wall chamber of the related art shown in FIG. 1. It shouldalso be noted that the exact orientation of the parallel chamber wallsis unimportant. For example, they may be at an angle of less than 90degrees to the circuit board to which the encapsulated device attaches.In this case, the chambers 55 may be slanted to form a low profileencapsulation device.

After the bare dice 76 have been inserted into the desired chambers 55,the cap 70 is seated on lip 75 (see FIG. 1) and retained and sealed tothe chamber portion 50 with a sealant glue. The cap contains acompression pad 130 which provides a flexible bias to the bare dice 76.Various substances may be used for the compression pad, including avariety of springs, gels, or foams. Because of the flexible bias, thecompression pad 130 biases the bare dice firmly into the retainingcontact. Therefore, the compression pad 130 of the cap 70 helps toeliminate breakage of the bare dice, even with jarring. The seating ofthe cap 70 on the lip 75 completes the encapsulation process.

The skilled artisan will appreciate that the compression pad 130 notonly serves die-retaining and shock-absorbing functions, but also hasthe potential to provide a significant capability to transfer heat awayfrom the dice 76. In the absence of some structure to contact the top ofthe dice 76, the paths for conductive heat transfer will be limited tothose areas of contact between the chamber 55 and the dice 76, at thesurfaces 135 and 140, and at the interface between the spring portion 86and the dice 76 as shown in FIGS. 4 and 5. However, the biasing contactbetween the elastomeric compression pad 130 and the top of the tie 76may provide a significant conductive heat transfer pathway, dependingupon the materials selected for the compression pad 130 and the cap 70.

In an embodiment, the compression pad 130 may be advantageously formedfrom an electrically-insulating, elastomeric material, such as asilicone-based jell that is impregnated with particles of a thermallyconducting material, such as diamond, beryllium oxide, silicon nitride,or like materials. The silicone-based elastomeric material is typicallya three-part mixture of a base material, a hardening material, and adiluent material. The Shore-A or Durometer value for the elastomericmaterial is influenced by the amount of hardening material—a higheramount of hardening material results in a higher Shore-A or Durometervalue. The diluent material influences the viscosity of the mixture.Silicone-based materials are preferred due to their high elasticity anddesirable electrical insulating properties. If relatively hightemperatures are anticipated, the compression pad 130 may be fabricatedfrom a polyamide silicone-based material. The mean size of therelatively small conducting particles, as well as their concentration inthe elastomeric material, is a matter of discretion on the part of thedesigner. Relatively smaller sized particles in the range of 5 to 10microns are preferred since the thermal conductivity of the conductingparticles normally does not diminish with particle size, and relativelylarger particles may damage the dice 76.

Depending upon the amount of heat dissipation required, differentcombinations of materials for the compression pad 130 and the cap 70 maybe selected. For example, the cap 70 may be fabricated from moldedplastic, and the compression pad 130 may be fashioned from asilicone-based jell with a Shore-A value of about 45 that is impregnatedwith diamond particles. If higher heat dissipation is necessary, the cap70 may be fabricated from a material with a higher thermoconductivity,such as aluminum or other metallic material. A metallic cap 70ordinarily would be undesirable due to the potential forshort-circuiting between the cap 70 and the dice 76. However, theexcellent insulating properties of the elastomeric material permit theuse of a metallic material for the cap 70. In addition, if space andpower consumption are not unduly limited, other heat sink devices, suchas fins, may be incorporated into the cap 70 to provide additionalmaterial available for heat conduction and to provide a larger availablesurface area for convective heat transfer. In addition, a fan may beused to provide forced convection.

Alternatively, the compression pad 130 may be potted, that is, poureddirectly over the dice 76 and into the slots 55, thereby sealing theslots 55. This alternative provides the aforementioned die-retaining,shock-absorbing and heat dissipation advantages, however, it does renderthe installation of the dice 76 permanent.

The exact dimensions of the encapsulation device 45 will depend on thesize of the dice. In a preferred embodiment the encapsulation device 45is approximately 600 mils long, approximately 630 mils wide, andapproximately 270 mils high with the cap 70 in place, the cap 70 itself,being approximately 70 mils high. In this preferred embodiment, eachchamber 55 is approximately 570 mils long, slightly more thanapproximately 18 mils wide at its widest point, and approximately 215mils deep at its deepest point.

Although the invention has been described with respect to specificembodiments, the invention is limited only as claimed.

What is claimed is:
 1. A method for mounting a bare die, comprising theacts of: inserting the bare die into a chamber assembly in asubstantially upright orientation on relative to a base of the chamberassembly; contacting an exposed portion of the bare die with acompression pad to facilitate cooling and flexible mounting of the baredie, wherein the compression pad comprises an elastomeric materialcontaining thermally conducting particles.
 2. The method of claim 1,wherein the act of inserting the bare die comprises the acts of:positioning an insertion tool adjacent the bare die; and supporting thebare die with the insertion tool during insertion of the bare die intothe chamber assembly.
 3. The method of claim 2, wherein the act ofsupporting the bare die comprises the act of: securing the bare die tothe insertion tool via a vacuum assembly coupled to the insertion tool.4. The method of claim 1, wherein the elastomeric material comprises asilicone based gel.
 5. The method of claim 1, wherein the thermallyconducting particles comprise diamond.
 6. The method of claim 1,comprising the act of: pressurably biasing the compression pad towardthe exposed portion.
 7. The method of claim 6, wherein the act ofpressurably biasing the compression pad comprises the act of: removablycoupling the compression pad to a top of the chamber assembly.
 8. Themethod of claim 1, comprising the acts of: coupling a flexible footassembly to the base of the chamber assembly; and electrically couplingthe bare die to the flexible foot assembly.
 9. The method of claim 8,comprising the acts of: positioning the chamber assembly adjacent acircuit; and electrically coupling the flexible foot assembly to thecircuit.